1. Field of the Invention
The present invention relates to a process for cleaning short mineral fibers that are contaminated with foreign bodies, which process uses a separation apparatus having a filter medium with slot-like recesses.
2. The Prior Art
Short fibers have an average length typically of 0.1 to 5 mm and an average diameter of approximately 0.1 to 10 .mu.m. The foreign bodies are predominantly non-fiber-like, nearly spherical particles, or fragments thereof, of the same material as the fibers, which originate as so-called "shots" with a diameter between about 50 and 2,000 .mu.m through the formation of droplets in the manufacture of the fibers, for example from highly viscous liquid in the free jet process.
Fiber composite materials include a fiber-reinforced metal matrix. When these fiber composite materials produced therefrom are cyclically stressed, shots of a size starting with a diameter of from 50 to 100 .mu.m cause cracking, especially within the metal matrix. In this way, the shots affect the otherwise good mechanical properties of fiber-reinforced metal matrix (for example, those used for the manufacture of pistons for internal-combustion engines) in an unfavorable way. For this reason, it is desirable to completely separate these shots from the fibers through suitable cleaning methods.
A known process for cleaning the fibers is the floatation of a preferably aqueous crude fiber suspension. In this process, the fibers are separated from the foreign bodies as a result of their greater specific surface area per unit volume. The drawback with this prior art process is that no complete separation of the fibers from the foreign bodies can be achieved. It is conceivable, for example, that shots contains in fiber balls are discharged together with the clean fiber suspension.
Another known process is the cleaning of the fibers through filtration of a preferably aqueous crude fiber suspension. As compared to floatation, complete separation of the mixture to be cleaned can be achieved through filtration. However, a problem with this prior art process is that there is always the filtration of fibers having a length greater than, for example, the width of the mesh of the sieve used for the filtration. The problem develops because after only a short time, a filter cake forms, which holds back both fibers and foreign bodies, which causes the separation process to stop. This problem can be resolved by the known methods of filtration only in an unsatisfactory way. As a result, a cleaning through filtration is, in the last analysis, possible only in connection with a mechanical damaging especially of the longer and longest fibers contained in the crude suspension. These fibers can break to a considerable extent, and may be partly obtained in powder form. In any case, the fibers cleaned by filtration are clearly shorter than in the starting condition.